Outer layers of plants such as leaf cuticles, fruit peels, and bark protect the plant against abrasion, prevent water loss, and protect against pathogenic microorganisms. Breaking through the plant protective outer layer is a prerequisite for a pathogen to enter the plant's internal tissues. Some studies have suggested that penetration of the protective layer involves dissolution of the host cuticle by enzymes secreted by the pathogen. Nicholson, R. L. et al., in The Fungal Spore and Disease Initiation in Plants and Animals, eds. Cole, G. T., and Hoch, H. C., 1991, Plenum Press, New York, pp. 3–23.
Pentacyclic triterpenes are among the most common plant secondary metabolites, but their function in plants has not been fully understood. They are usually concentrated in the outermost layers such as plant cuticle, fruit peel, and bark.
Betulin is a pentacyclic triterpenoid derived from the outer bark of paper birch trees (Betula papyrifera, B. pendula, B. verucosa, etc.). It can be present at concentrations of up to about 24% of the bark of white birch. Merck Index, twelfth edition, page 1236 (1996). Lupeol is a related compound also found in birch bark and in other plant sources. Lupeol is present at concentrations of about 1.5–3% of the birch bark and at up to about 8.2% in Canavalia ensiformis, a plant widespread in the humid tropics of Asia and Africa. Allobetulin is another triterpenoid found in birch bark. A typical pulp mill that process birch produces enough bark waste to allow for the inexpensive isolation of significant quantities of these triterpenoids.
Literature supplies examples of enzymes that can be inhibited by triterpenes, indicating the ability of triterpenes to act broadly in a non-specific mode on multiple targets. For example, Buchler et al. (Biochem. Biophys. Acta 1075, 206 (1991) showed inhibition of rat renal 11β-hydroxysteroid dehydrogenase. Koch et al. (Phytother, Res. 8, 109 (1994)) showed in vitro inhibition of adenosine deaminase. This leads to the hypothesis that pentacyclic triterpenoids in plant protective outer layers may protect against infection by inhibiting enzymes that would degrade the cuticle.
Several triterpenoids have been found to have utility. For example, betulin and related compounds have been shown to have anti-viral activity against herpes simplex virus. Carlson et al., U.S. Pat. No. 5,750,578. Betulin and related compounds have also been shown to have anti-fungal and anti-bacterial activity. However, triterpenoids are hydrophobic compounds with relatively low interfacial activity and water solubility. For instance, the solubility of betulin in water is about 0.15 mg/l. The relatively low interfacial activity and water solubility can make handling and administration of the compounds difficult. Low interfacial activity also limits the efficient interaction with target (fungi or bacteria) cell membranes. It also limits accessibility to hydrophilic biological targets or targets protected by a hydrophilic barrier.
There is a need for new compounds with anti-fungal, and anti-bacterial activities. Preferably the compounds would be relatively water soluble surfactants. The water-soluble compounds would retain their utility, e.g., antifungal, antibacterial, etc., activity.